A glass preform made of silica glass containing substantially no transition metal such as iron is advantageously produced by the vapor-phase axial deposition method (hereinafter referred to as "VAD" method), which comprises flame hydrolyzing a halide of of Si, Ge, B or P in an oxyhydrogen flame and depositing formed fine glass particles on a seed member to obtain a fine glass particle mass, namely a porous soot preform.
The VAD method is suitable for the production of the glass preform for use in the economical fabrication of an optical fiber having low attenuation of light transmission, arbitrary distribution of refractive index in its radial direction and homogeneous composition in its longitudinal direction and on its circumference. According to the VAD method, the glass preform is produced as follows:
Fine glass particles are formed by flame hydrolysis of a starting glass material and then deposited on a rotating seed member such as a glass plate or rod in the flame to form a cylindrical fine glass particle mass with an adequate refractive index distribution in the radial direction. The fine glass particle mass is then sintered at a high temperature to obtain a transparent glass preform.
Advantages of the VAD method are that the yield of the starting glass material is good, that the glass preform contains less impurities except hydroxyl groups, that the production time is short, that the distribution of the refractive index is easily controlled, and that the method includes fewer steps. Therefore, the VAD method is valuable for mass production of the optical fiber.
However, the glass preform inevitably contains unreacted water in an amount of 30 to 70 ppm since the VAD method includes hydrolysis.
Recently, it is increasingly desired to use a wavelength range near 1.3 micrometer at which absorption loss due to structural imperfection is lowest for optical transmission purpose. Since the absorption loss due to the residual hydroxyl groups is, however, serious in this wavelength range, it is necessary to decrease the amount of the hydroxyl groups in the optical fiber to 0.3 ppm or less.
For this end, it is proposed to remove water in the fine glass particle mass, from which the glass preform is produced by sintering, by decomposing water with chlorine (Cl.sub.2) or a chlorine-containing compound (e.g. SOCl.sub.2) to hydrogen chloride and oxygen (cf. Japanese Patent Publication Nos. 40096/1982 and 13503/1983). It is possible to considerably effectively remove water form the glass preform and to decrease the amount of the residual hydroxyl groups in the optical fiber to 0.1 ppm or less by using a dehydrating agent comprising chlorine or the chlorine-containing compound.
However, chlorine or the chlorine-containing compound not only acts as the dehydrating agent but also reacts with an additive for controlling the refractive index of glass (e.g. GeO.sub.2, P.sub.2 O.sub.5, etc.) and SiO.sub.2 to form many structural defects in the glass preform. When an optical fiber is fabricated from the glass preform with structural defects, hydrogen gas, which is present in air in an amount of about 0.01% by mole, acts on the structural defects to form the hydroxyl groups. The thus formed hydroxyl groups increase the attenuation of light transmission and sometimes make it impossible to transmit light through the optical fiber.
It has been found that the number of the structural defects increases as the amount of the additive in the glass preform increases. From this fact, the reformation of the hydroxy group may proceed according to the following equation: EQU GeO(defect)+1/2H.sub.2 .fwdarw.GeOH (I)
FIG. 1 shows change of attenuation of light transmission at various wavelength of an optical fiber which is fabricated from a glass preform dehydrated in the absence of oxygen. In FIG. 1, the chained line, the broken line and the solid line stand for the initial attenuation, that after 11 months and that after 16 months, respectively. From these results, it is seen that the absorption at a wavelength near 1.4 micrometer due to the presence of the hydroxyl groups increases as the time passes.